66. Particles Must be very Near to Each Other to Adhere.—Why is it that when you have broken any thing made of glass, however accurately you may bring the two parts together, you can not make them unite in one again? It is simply because the particles of substances will not attract each other enough to be united unless they are brought very near together. Now it is impossible to bring the particles on the two surfaces of a broken piece of glass as near together as they were before it was broken. If you could do so, no crack would be visible. You can join them by some kind of cement. This is because the particles of the cement, while it is soft, can be insinuated among the particles of the glass; and thus, when it dries, it becomes a bond of union between the particles on each side of the breach. For the same reason you can make the cut surfaces of some yielding substances adhere. If you divide a piece of India-rubber with a clean cut, you can make the two surfaces adhere by pressing them together firmly. The particles in this case are not unyielding, as those of the glass are, and some of them are therefore brought into such near neighborhood as to attract each other sufficiently to unite together. So, too, if you cut two bullets so as to have a very smooth flat surface on each, you can make them adhere quite strongly by pressing them together, especially if you give a little turning motion at the same time that you press, for this will cause the particles on the two surfaces to be somewhat mingled together. If you have quite large balls of lead with handles, as represented in Fig. 14, it will require considerable force to separate them when they have been thus pressed together.
67. Other Illustrations.—Silver and gold may be made to adhere to iron by a very great and sudden pressure. The iron must be made very smooth, and the silver or gold plate very thin. A powerful blow brings the particles of the thin plate into such nearness to those of the iron that union is effected, or, in other words, that they attract each other sufficiently to be united. So, also, a sheet of tin and one of lead can be made to adhere so as to form one sheet by the pressure of the rollers of a flatting-mill. Two very smooth panes of glass laid one upon another may have so many particles brought into great nearness as to occasion some adhesion. It will be slight, however, few comparatively of all the particles coming near enough to adhere, for the smoothest glass is full of inequalities, as may be seen by the microscope.
68. Strength of Adhesion.—In no case do particles come in actual contact (§ 20), and their adhesion depends on the nearness of their neighborhood to each other. The strength of union, then, between two surfaces depends on the number of particles brought near enough to adhere together. In the case of the two bullets or the lead balls, if all the particles of the two surfaces were near enough to adhere, the lead would be just as strong at the junction as any where else. The reason that so strong adhesion takes place between portions of some substances when we soften them by heat is that the particles of the two softened ends are all brought near enough together for adhesion. Thus the two ends of a broken stick of sealing-wax may be firmly united by heating them and then pressing them together. The same thing can be done with glass. When iron is welded, as it is termed, some hammering is required to make the particles of the two softened ends of the iron unite.
Fig. 15.
69. Attraction Between Solids and Liquids.—The attraction which solids and liquids have for each other furnishes us with many interesting phenomena. The adhesion of drops of water to glass and other solids is a familiar example of this attraction. If you dip your hand into water, it is wet on taking it out, because your skin has sufficient attraction for the water to retain some of it. A towel will retain more of it for two reasons—with the interstices between its fibres it presents much more surface to the water, and it has none of the oily substance which on your skin, though being in small quantity, serves somewhat to repel the water. The attraction of solids and fluids for each other is shown very prettily in the experiment represented in Fig. 15 (p. 47). A piece of wood is attached by the string, a, to one end of a balance, and weights just sufficient to balance it are placed in the opposite scale. If now the wood is brought in contact with the water in the vessel, b, it will require additional weight in the scale to separate the wood from the water.
Fig. 16. Fig. 17.
Fig. 18.